The present invention relates to digital imaging and, more specifically, to image-scanning systems.
Much of modern progress is associated with the increasing prevalence of computers. Computers typically manage information in the form binary values embodied as voltage values. Since this form of information is not directly accessible to humans, peripherals that translate between the human perceptual world and the digital world of computers have also become increasingly prevalent. For example, printers convert digital image data to human-readable images, and scanners convert human-readable images to digital data.
Many scanners convert an image to digital form raster (line-by-line) fashion. Each line is digitized using a linear array of photo-sensors that extends across some or all of the width of the image. For example, some scanners use a contact image sensor (CIS) array. The CIS array illuminates an image with light-emitting diodes (LEDs), uses a lens array to set a focal distance, and senses the light with CMOS photo-sensors. Each photo-sensor characterizes one pixel of an image line. The array is scanned so that each photo sensor traverses the length of the image. The output of each sensor is sampled so that the collective output of the array is a digital representation of the image. The digital representation can be a 1-bit monochrome representation, a multi-bit grey-scale representation, or a multi-bit color representation.
Unfortunately, the CIS is not a perfect sensor. Sensor gain (the amount a sensor's electrical output varies for a given change in its photo input) and offset (the output level when there is no illumination) vary from sensor to sensor, and may degrade over time. The LED light sources vary from unit to unit and will change intensity over long periods of time (years). The optics have imperfections and shadows. The glass surface can become scratched or contaminated. To compensate for imperfections in light sources, optics, sensors, and electronics, the CIS must be calibrated before a good scan/copy can be made, and may require user re-calibration.
In some cases, a sensor is so far out of specifications that calibration alone does not yield satisfactory performance. One sensor that is far out of specification can result in a calibration that causes the remaining sensors to operate out of an optimal range, resulting in poor overall image quality. In extreme cases, one or more “dead” sensors may fail to respond to variations in light intensity: a permanently “off” sensor may indicate “dark” in the presence of light; or, a permanently “on” sensor may indicate “light” even in the absence of light.
A dead or otherwise defective sensor can result in an artifact over the length of an image. For example, an always-off pixel can result in a dark vertical line in the digital image, while an always-on pixel can result in a light vertical line in the digital image. If this condition is detected at the manufacturer, the usual solution is to replace the sensor array. However, if the problem develops at the user's site, then an inconvenient and expensive repair can be required.
What is needed is an approach for handling defective sensors, especially when the defect is developed in use. Preferably, satisfactory scanning should be achievable even with the presence of one or more defective sensors.